This invention pertains generally to broadband circularly polarized spiral antennas and particularly to an improved feed for such type of antennas.
As is known in the art, a multi-arm spiral antenna is used to advantage in a direction finder system because such an antenna can encode the location of a radiation emitter in information contained in its sum and difference beams and is circularly polarized and relatively broadband. A typical spiral antenna for such an application is a four-arm device, with each one of the arms fed by a separate wire of a four-wire balanced transmission line. Theoretically, a relatively good impedance match over the desired bandwidth may be maintained between the antenna elements and the associated four-wire feed for any possible radiating mode of the antenna. When a monopulse arithmetic network is used to develop sum and difference mode direction-finding signals such a network usually is a transverse electro-magnetic (TEM) mode device as might be fabricated in stripline so that an interface is required between the four-wire feed for the radiating arms of the antenna and the monopulse arithmetic network. Unfortunately, when a four coaxial line to four-wire transmission line transition is required, the characteristic impedances of the two kinds of transmission line differ substantially and impedance mismatch causes a large reflection so the usefulness of the resulting systems is impaired by this contribution to direction finder angle measurement error.
One conventional technique for realizing the requisite transition is to form a junction between the four coaxial cables from the monopulse arithmetic network and the spiral antenna after stripping away a portion of the outer conductors of each one of the four coaxial cables so that the free ends of each such cable together form a four-wire transmission line. The outer conductors of each one of the coaxial cables are soldered into a restraining yoke located near the junction of the four-wire feed to the spiral antenna radiating arms and the free ends of the four-wire line formed by the center conductors are soldered to arms of the spiral antenna to connect each arm to the monopulse arithmetic network. Unfortunately, however, a transition so formed exhibits a relatively high voltage standing wave ratio (VSWR) over the entire operating bandwidth for both the sum and difference radiating modes of the antenna actually used. As a result, then, phase and amplitude errors are introduced in the operation of a direction-finder antenna system using such a known transition.